A rare-earth-iron-boron based rare-earth magnet is a typical high-performance permanent magnet, has a structure including, as a main phase, an R2Fe14B-type crystalline phase, which is a ternary tetragonal compound, and exhibits excellent magnet performance. In R2Fe14B, R is at least one element selected from the group consisting of the rare-earth elements and yttrium and portions of Fe and B may be replaced with other elements.
Such rare-earth-iron-boron based rare-earth magnets are roughly classifiable into sintered magnets and bonded magnets. A sintered magnet is produced by compacting a fine powder of a rare-earth-iron-boron based magnet alloy (with a mean particle size of several μm) with a press machine and then sintering the resultant compact. On the other hand, a bonded magnet is usually produced by compacting a mixture (i.e., a compound) of a powder of a rare-earth-iron-boron based magnet alloy (with particle sizes of about 100 μm) and a binder resin within a press machine.
The sintered magnet is made of a powder with relatively small particle sizes, and therefore, the respective powder particles thereof exhibit magnetic anisotropy. For that reason, an aligning magnetic field is applied to the powder being compacted by the press machine, thereby obtaining a compact in which the powder particles are aligned with the direction of the magnetic field.
In the bonded magnet on the other hand, the powder particles used have particle sizes exceeding the crystal grain size, and normally exhibit no magnetic anisotropy and cannot be aligned under a magnetic field applied. Accordingly, to produce an anisotropic bonded magnet in which the powder particles are aligned with particular directions, a technique of making a magnetic powder, of which the respective powder particles exhibit the magnetic anisotropy, needs to be established.
To make a rare-earth alloy powder for an anisotropic bonded magnet, an HDDR (hydrogenation-disproportionation-desorption-recombination) process is currently researched and developed. The “HDDR” means a process in which hydrogenation, disproportionation, desorption and recombination are carried out in this order. In this HDDR process, a cast flake or powder of a rare-earth-iron-boron based alloy is maintained at a temperature of 500° C. to 1,000° C. within an H2 gas atmosphere or a mixture of an H2 gas and an inert gas so as to absorb hydrogen. As a result of this hydrogen absorption, the R2Fe14B phase is decomposed into rare-earth hydrides and iron-based borides. This reaction is represented by any of the following chemical equations:R2Fe14B+2H22RH2+Fe2B+12Fe orR2Fe14B+2H22RH2+Fe3B+11Fe
Thereafter, the hydrogenated flake or powder is subjected to a desorption process at a temperature of 500° C. to 1,000° C. and then cooled, thereby obtaining an alloy magnet powder. As a result of this desorption process, the R2Fe14B phase is regenerated from the hydrides or iron-based borides described above.
The respective R2Fe14B crystal grains, which had relatively large grain sizes (of several tens of μm or more, for example) before subjected to the hydrogenation process, turn into an aggregation of a huge number of very small R2Fe14B crystal grains (with grain sizes of approximately 0.1 μm to 1 μm). An aggregation of very small R2Fe14B crystal grains obtained in this manner will be referred to herein as a “recrystallized texture”. The very small R2Fe14B crystal grains in the recrystallized texture retain the crystallographic orientations of the original big R2Fe14B crystal grains. Accordingly, if the HDDR processed alloy powder is subjected to pulverization, classification and other processes such that its particle sizes are decreased to the sizes of the crystal grains yet to be subjected to the HDDR process or less, then the crystallographic orientations of those very small R2Fe14B crystal grains included in the respective powder particles can be aligned with a particular direction, thus realizing magnetic anisotropy. Also, the very small R2Fe14B crystal grains in the “recrystallized texture” have sizes that are close to the single domain critical grain size, thus achieving high coercivity, too.
Hereinafter, the HDDR process will be described with reference to FIGS. 19(a) through 19(e).
FIG. 19(a) schematically illustrates a portion of a rare-earth-iron-boron based master alloy 1. Since the master alloy 1 is polycrystalline, there are a lot of grain boundaries 3 and not all of the crystallographic orientations 2 of its crystal grains are aligned with each other. Thus, the master alloy 1 is subjected to a coarse pulverization process, thereby forming powder particles 5, each of which is big enough to have a single crystallographic orientation as shown in FIG. 19(b). If the powder particles 5 have excessively large particle sizes, then each of those particles 5 will become polycrystalline and the orientations of the crystal grains included in each powder particle 5 will not be aligned with each other. A set of those powder particles 5 will be referred to herein as a “coarsely pulverized powder” 4.
Next, the coarsely pulverized powder 4 is subjected to an HDDR process, thereby giving each particle 5 the recrystallized texture. FIG. 19(c) illustrates a state in which the recrystallized texture 7 has been formed in each powder particle 5. FIG. 19(d) is an enlarged view of the recrystallized texture 7, showing that the crystallographic orientations 2 of the respective crystal grains are aligned with each other in the texture.
Subsequently, as shown in FIG. 19(e), the powder particles 5 are either disbanded or finely pulverized, thereby obtaining an alloy powder 9 with magnetic anisotropy.
A method of making a rare-earth-iron-boron based alloy powder with the recrystallized texture by performing such an HDDR process is disclosed in Japanese Patent Gazettes for Opposition No. 6-82575 and No. 7-68561, for example.
The magnetic powder prepared by the HDDR powder (which will be referred to herein as an “HDDR powder”), however, has the following drawbacks.
Firstly, to increase the remanence of the HDDR powder, the master alloy must be subjected to a homogenizing process to be carried out at an elevated temperature for a long time (e.g., at 1,100° C. for 20 hours). This process is required because if the master alloy has a fine texture, then the material powder yet to be subjected to the HDDR process will become polycrystalline and the powder particles will become magnetically isotropic.
Also, to subject the master alloy to the HDDR process in its entirety, hydrogen needs to be sufficiently diffused so as to reach the inside of the master alloy. To do so, the hydrogenation process must be carried out for a rather long time (e.g., at 800° C. for six hours). However, the longer the hydrogenation process time, the lower the saturation magnetization tends to be. The reason is as follows. Specifically, as the hydrogenation process lingers, the reversible reactions as represented by the above chemical formulae repeatedly occur an increasing number of times. Then, the crystallographic orientations of the R2Fe14B phase, which are retained in the master alloy, are gradually lost. As a result, the resultant “recrystallized texture” will have decreased magnetic anisotropy.
Nevertheless, if the hydrogenation process time is shortened, then the HDDR process will be incomplete, an insufficient amount of fine R2Fe14B phase will be produced, and both the coercivity HcJ and remanence Br will drop.
To overcome this problem, according to a proposed technique, Ga or any other element is added to the master alloy. In particular, by adding Ga to the master alloy, even if the hydrogenation process is carried out for a rather long time, the crystallographic orientations of the R2Fe14B phase, retained in the master alloy, are not lost so easily. As a result, both the coercivity HcJ and remanence Br can be increased to sufficient levels.
However, Ga is an expensive material, and the extended heat treatment for the purpose of hydrogenation increases the manufacturing cost, too. Thus, to mass-produce an HDDR powder at a reduced cost with the properties of the HDDR powder improved, the addition of that expensive Ga needs to be avoided and yet required magnet performance needs to be achieved in a short hydrogenation process time.
In Japanese Patent Gazettes for Opposition No. 6-82575 and No. 7-68561 identified above, a so-called alloy ingot, obtained by melting and casting a material alloy using an induction melting crucible, is used as the master alloy. Recently, however, a method for making a bonded magnet from a powder obtained by subjecting a strip-cast thin-plate material (or alloy flakes) to the HDDR process was also proposed (see Japanese Patent No. 3213638, for example).
However, the strip-cast alloy flakes include substantially no α-Fe phases, have a homogenous texture, but have excessively small crystal grain sizes. Thus, at a powder particle size applicable to a bonded magnet, the respective powder particles exhibit too low magnetic anisotropy to use the powder effectively.
In order to overcome the problems described above, a primary object of the present invention is to provide a rare-earth-iron-boron based alloy, which can eliminate the homogenizing process of the master alloy, can shorten the hydrogenation process time, and can improve both the coercivity HcJ and remanence Jr alike even substantially without adding Ga, and also provide a magnetically anisotropic magnet powder and a method of making the powder and an anisotropic bonded magnet and a method for producing the magnet.